1. Field of the Invention
The invention relates to a vehicle, a power transmitting device and a contactless power supply system and, more particularly, to a technique for improving power transfer efficiency in a contactless power supply system.
2. Description of Related Art
In recent years, contactless wireless power transfer that does not use a power cord or a power transmission cable has become a focus of attention, and it has been suggested that the contactless wireless power transfer is applied to an electric vehicle, a hybrid vehicle, or the like, of which an in-vehicle electrical storage device is chargeable by a power supply outside the vehicle (hereinafter, also referred to as “external power supply”).
In a contactless power supply system, in order to improve power transfer efficiency, it is important to match impedance between a power transmitting side and a power receiving side.
At the power receiving side of such a contactless power supply system, generally, an electrical storage device for storing received electric power is provided. As the state of charge of the electrical storage device varies, the input impedance of the power receiving side can vary accordingly.
Japanese Patent Application Publication No. 2011-120443 (JP 2011-120443 A) describes a contactless power transfer system that is configured to carry out impedance matching by varying the duty of a DC/DC converter, which converts received electric power and supplies the converted electric power to a battery, in synchronization with a variation in input impedance due to a load fluctuation in response to the state of charge of the battery.
With the technique described in JP 2011-120443 A, it is possible to suppress a decrease in power transfer efficiency due to a mismatch of impedance by adjusting the input impedance, which can vary with the state of charge of the battery, with the use of the DC/DC converter.
However, in order to make it possible to carry out impedance matching over all the fluctuation range of impedance, the DC/DC converter used is required to have specifications (rated power capacity, or the like) compatible with such impedance matching. This increases the size of the DC/DC converter itself, and also increases required cost. Furthermore, when the size of the DC/DC converter increases, a loss in driving the DC/DC converter also increases, so there is a concern that overall power transfer efficiency may also be influenced.